1. Field of the Invention
The present invention relates to resists used in photolithography and more particularly, to an improved X-ray and electron sensitive resist material having increased specific absorption so as to provide enhancement of the lithographic processing of microcircuits.
2. Description of the Related Art
Resists for all modes of high resolution lithography require chemical changes induced by free electrons under control of an incident illumination or pattern writing beam. Free electrons are important in the exposure of the resist, regardless of whether the incident beam is formed by electrons, ions, or ionizing photons. Thinner resists with higher sensitivity are desirable for high resolution lithography.
The exposure levels and absorption in the resist thickness are set by the properties of the resist and the absorption statistics required to achieve pattern definition with high confidence. The resist must be exposed throughout its thickness. As a result of the requirement that the deepest layers of resist receive the exposure beam, and the fact that no thin layer is totally absorbing for any type of exposure beam, it is physically impossible for a resist to be both fully exposed and totally absorbing. Thus, some transmitted exposure beam will, of necessity, penetrate into the substrate, generally with some deleterious residual effects on the substrate.
Each absorption event in the resist, i.e., each event in which an incident beam particle or photon is absorbed, can expose a small volume element of resist. In a resist with low photoelectric absorption, many incident photons must pass through in order to produce absorptions and exposures in all resolvable volume elements. When the specific absorption of the resist is increased, fewer incident photons are required to produce absorption events and exposures of each volume element. The same statistics are required for absorbed events in both the lower and higher absorption types of resist, but the higher absorption resist achieves these statistics with fewer incident photons transmitted to the substrate. The reduced exposure requirements translate directly into reduced resist thickness, enhanced process throughput, or reduced radiation exposure to the resist and substrate.
Prior art efforts have sought to increase the X-ray absorption of resists by altering the molecular constituents of the resist to include higher atomic number elements such as sulfur and others. It is well known that higher atomic number elements increase the cross section for absorption of the incident beam. Adding new elements to resist polymers can only be accomplished as the corresponding chemistry allows, and can only be carried out to low concentration levels. Extensive investigations into this approach have involved significant levels of synthetic chemistry operating on the very molecules whose radiation chemistry and physical properties, not just radiation absorption, may also be affected. Altering the polymer chemistry alters the carefully developed polymer properties, a subject about which much is known in the field. (See C. P. Wong, editor, Polymers for Electronic and Photonic Applications, Academic Press, NY, 1993; Theodore Davidson, editor, Polymers in Electronics, American Chemical Society, Washington, D. C., 1984; and Eugene D. Feit, and Cletuw W. Wilkins, Jr., Editors, Polymer Materials for Electronic Applications, American Chemical Society, Washington, D.C., 1982.)
Within the field of X-ray lithography, a local dose enhancement effect (increase in exposure) at the resist/substrate interface has previously been observed by many workers and is due to the high atomic number substrate. (See D. J. D. Carter, A. Pepin, M. R. Schweizer et al., “Direct measurement of the effect of substrate photoelectrons in x-ray nanolithography,” J. Vac. Sci. technol. B 15 (6), 2509-13 (1997).) As will appear, the present invention, in essence, puts this “substrate effect” to practical use by distributing the high atomic number absorber throughout the volume of the resist in a manner that is chemically and physically tractable.